Fuel ethanol is currently produced from feedstocks such as corn starch, sugar cane, and sugar beets. However, the potential for production of ethanol from these sources is limited as most of the farmland which is suitable for the production of these crops is already in use as a food source for humans. Furthermore, the production of ethanol from these feedstocks has a negative impact on the environment because fossil fuels used in the conversion process produce carbon dioxide and other byproducts.
The production of ethanol from cellulose-containing feedstocks, such as agricultural wastes, grasses, and forestry wastes, has received much attention in recent years. The reasons for this are that these feedstocks are widely available and inexpensive and their use for ethanol production provides an alternative to burning or landfilling lignocellulosic waste materials. Moreover, a byproduct of cellulose conversion, lignin, can be used as a fuel to power the process instead of fossil fuels. Several studies have concluded that, when the entire production and consumption cycle is taken into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The three primary constituents of lignocellulosic feedstocks are cellulose, which comprises 30% to 50% of most of the key feedstocks; hemicellulose, which comprises 15% to 35% of most feedstocks; and lignin, which comprises 15% to 30% of most feedstocks. Cellulose and hemicellulose are comprised primarily of carbohydrates and are the source of sugars that can potentially be fermented to ethanol. Lignin is a phenylpropane lattice that is not converted to ethanol.
Cellulose is a polymer of glucose with beta-1,4 linkages and this structure is common among the feedstocks of interest. Hemicellulose has a more complex structure that varies among the feedstocks. For the feedstocks which are typically of interest, the hemicellulose typically consists of a backbone polymer of xylose with beta-1,4 linkages, with side chains of 1 to 5 arabinose units with alpha-1,3 linkages, or acetyl moieties, or other organic acid moieties such as glucuronyl groups.
The first process step for converting lignocellulosic feedstock to ethanol involves breaking down the fibrous material. The two primary processes are acid hydrolysis, which involves the hydrolysis of the feedstock using a single step of acid treatment, and enzymatic hydrolysis, which involves an acid pretreatment followed by hydrolysis with cellulase enzymes.
In the acid hydrolysis process, the feedstock is subjected to steam and a mineral acid, such as sulfuric acid, sulfurous acid, hydrochloric acid, or phosphoric acid. The temperature, acid concentration and duration of the acid hydrolysis are sufficient to hydrolyze the cellulose and hemicellulose to their monomeric constituents, which is glucose from cellulose and xylose, galactose, mannose, arabinose, acetic acid, galacturonic acid, and glucuronic acid from hemicellulose. If sulfuric acid is employed, it can be concentrated (25-80% w/w) or dilute (3-8% w/w). The resulting aqueous slurry contains unhydrolyzed fiber that is primarily lignin, and an aqueous solution of glucose, xylose, organic acids, including primarily acetic acid, but also glucuronic acid, formic acid, lactic acid and galacturonic acid, and the mineral acid. Although this process produces ethanol, the yield is low due to the non-selective nature of the acid hydrolysis.
In the enzymatic hydrolysis process, the steam temperature, mineral acid (typically sulfuric acid) concentration and treatment time of the acid pretreatment step are chosen to be milder than that in the acid hydrolysis process. Similar to the acid hydrolysis process, the hemicellulose is hydrolyzed to xylose, galactose, mannose, arabinose, acetic acid, glucuronic acid, formic acid and galacturonic acid. However, the milder pretreatment does not hydrolyze a large portion of the cellulose, but rather increases the cellulose surface area as the fibrous feedstock is converted to a muddy texture. The pretreated cellulose is then hydrolyzed to glucose in a subsequent step that uses cellulase enzymes.
Prior to the addition of enzyme, the pH of the acidic feedstock is adjusted to a value that is suitable for the enzymatic hydrolysis reaction. Typically, this involves the addition of alkali to a pH of between about 4 and about 6, which is the optimal pH range for cellulases, although the pH can be higher if alkalophilic cellulases are used and lower if acidic cellulases are used. Alkali that are most commonly used to adjust the pH of the acidified pretreated feedstock prior to hydrolysis by cellulase enzymes are ammonia, ammonium hydroxide and sodium hydroxide, although the use of carbonate salts such as potassium carbonate, potassium bicarbonate, sodium carbonate and sodium bicarbonate has also been contemplated. (See WO 2006/128304, Foody et al.).
U.S. Pat. No. 5,628,830 (Brink) discloses the use of calcium carbonate to adjust the pH of an aqueous sugar solution containing xylose, glucose, mannose and galactose arising from acid hydrolysis of lignocellulosic feedstock. After pH adjustment of the aqueous sugar solution, the solution is submitted to fermentation. However, Brink's process employs full acid hydrolysis, which suffers from the disadvantage discussed above.
One shortcoming of processing lignocellulosic feedstocks to produce glucose is the large amounts of alkali that are required to adjust the pH of the acid pretreated feedstock prior to enzymatic hydrolysis with cellulase enzymes. The addition of alkali adds significant cost to the process. In addition, the alkali reacts with the acid to produce salt, which must be processed or disposed of.
U.S. Pat. No. 4,425,433 (Neves) discloses the use of sodium carbonate or sodium bicarbonate to neutralize an acidic feedstock slurry containing glucose, which slurry is produced by acid hydrolysis of the cellulose and hemicellulose components of the feedstock. After the neutralization, the acidic slurry or “wort”, as referred to therein, is submitted to fermentation. However, a disadvantage of this process is that the amount of sodium carbonate and sodium bicarbonate required for the pH adjustment would add significant cost to the process and produce a large amount of salt to be disposed of.
U.S. Pat. No. 6,927,048 (Verser et al.) discloses a process in which calcium carbonate and an amine or an alcohol are added during the fermentation of glucose to acetic acid. The calcium carbonate controls the pH while the amine or alcohol complexes with the acetic acid. After the fermentation, the calcium carbonate is precipitated by the addition of carbon dioxide and then recovered from the fermentation broth. The recovered calcium carbonate is then reused in the subsequent fermentation. Thus, Verser et al. does not address the reduction of alkali use during the pretreatment and neutralization of a lignocellulosic feedstock.
U.S. Pat. No. 6,043,392 (Holtzapple et al.) also does not address reducing alkali usage during a neutralization conducted after acid pretreatment of a lignocellulosic feedstock. Rather, Holtzapple discloses a process that involves lime (alkali) treatment of lignocellulosic feedstocks with a subsequent fermentation step to produce volatile fatty acids (VFAs), followed by a thermal conversion of the VFAs to produce ketones. Calcium carbonate may be produced during an evaporation step involving carbon dioxide addition prior to thermal conversion of the VFAs. The calcium carbonate is recycled to the fermentor to neutralize acids that are produced by the fermentation or is burned in a lime kiln to produce lime which may be used in the lime treatment.
Similarly, U.S. Pat. No. 5,693,296, also to Holtzapple, discloses a process involving treating biomass with calcium oxide or hydroxide, followed by carbonating the pretreated material to form calcium carbonate or bicarbonate. The calcium carbonate may be heated in a lime kiln to form calcium oxide, which can be hydrated to form calcium hydroxide, which, in turn, can be used to treat the biomass. Thus, this process also does not address reducing chemical usage during a neutralization of an acid pretreated feedstock in the production of glucose. A similar process is disclosed by Chang et al., 1998, Applied Biochemistry and Biotechnology, 74:135-159.
US 2006/0188965 (Wyman and Lloyd) discloses a process involving acid pretreatment of cellulosic biomass. The acid-pretreated feedstock slurry is then mixed with a lime solution to impart a pH of 10 to 11, followed by the addition of sulfuric acid to adjust the pH into a range of 5-7 prior to cellulose hydrolysis by cellulase. Following the enzymatic hydrolysis, a fermentation of the hydrolyzed material is carried out to produce alcohol, which is then concentrated by distillation. Remaining liquids and/or solids from the distillation are subjected to a recycle processing step to filter fine particulates. The resulting material is then sent back to the acid pretreatment, along with lignocellulosic material fed to the process. However, the recycling of this material back to pretreatment does not reduce the amount of alkali used to neutralize the pretreated cellulose.
At present, none of the prior art addresses operating an efficient and economical process for hydrolyzing lignocellulosic feedstocks to glucose, while decreasing alkali usage. The development of an efficient process to decrease alkali usage remains a critical requirement to convert a lignocellulosic feedstock to glucose.